Nucleic Acid Biochemistry Practice Questions

Q: What enzyme deficiency is associated with the overproduction of uric acid and the development of gout?

Hypoxanthine-guanine phosphoribosyltransferase. ***Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)*** - A deficiency in **HGPRT** leads to an inability to salvage hypoxanthine and guanine, shunting these purine bases towards degradation. - This results in the **overproduction of uric acid** as the end product of purine metabolism, directly contributing to hyperuricemia and gout. *Adenosine deaminase* - Deficiency of **adenosine deaminase (ADA)** leads to the accumulation of deoxyadenosine and its derivatives, which are toxic to lymphocytes. - This causes **severe combined immunodeficiency (SCID)**, not gout or uric acid overproduction. *Xanthine oxidase* - **Xanthine oxidase** is the enzyme responsible for the final steps of purine degradation, converting hypoxanthine to xanthine and then xanthine to uric acid. - Inhibition of xanthine oxidase (e.g., by allopurinol) is a treatment for gout, meaning a **deficiency would *reduce* uric acid production**, not increase it. *Uricase* - **Uricase** is an enzyme that converts uric acid into allantoin, a more soluble compound. - Humans naturally lack this enzyme, which is why we excrete uric acid; its absence is normal and not a "deficiency" causing *overproduction* of uric acid, although some conditions leverage recombinant uricase for treatment.

Q: Which of the following is NOT a function of DNA polymerase I?

Not essential for bacterial survival.. ***Not essential for bacterial survival.*** - While DNA polymerase I is important, it is **not absolutely essential** for _E. coli_ survival because **DNA polymerase III** carries out the bulk of replication, and other repair enzymes can compensate for some of its repair functions. - "Not essential for bacterial survival" is a **characteristic/property**, not a **function** of the enzyme, making it the correct answer to this "NOT a function" question. - However, its absence does lead to a **mutator phenotype** and increased sensitivity to DNA damaging agents. *Involved in the synthesis of Okazaki fragments.* - DNA polymerase I **IS** involved in **Okazaki fragment processing** on the lagging strand. - It removes RNA primers using its **5' → 3' exonuclease** activity and fills the resulting gaps with DNA nucleotides. - The initial synthesis (elongation) of Okazaki fragments is carried out by **DNA polymerase III**, but Pol I completes their maturation. *Repairs damaged DNA.* - DNA polymerase I possesses both **5' → 3' exonuclease** and **3' → 5' exonuclease** activity, allowing it to remove damaged bases and incorporate new DNA. - It plays a crucial role in various DNA repair mechanisms, including **nucleotide excision repair** and **base excision repair**. *Participates in DNA replication.* - DNA polymerase I is essential for DNA replication, specifically in the **maturation of Okazaki fragments** on the lagging strand by removing RNA primers and filling gaps. - It also contributes to the **proofreading** and repair of newly synthesized DNA during replication.

Q: Which of the following statements about tRNA is correct?

The CCA sequence is added post-transcriptionally.. ***The CCA sequence is added post-transcriptionally.*** - The **CCA motif** at the 3' end of tRNA is crucial for amino acid attachment and is added by the enzyme **tRNA nucleotidyltransferase** after transcription. - This **post-transcriptional modification** ensures that the tRNA is fully functional for protein synthesis. *80% of total RNA* - **Ribosomal RNA (rRNA)**, not tRNA, constitutes the majority (approximately 80%) of cellular RNA. - tRNA typically makes up about **15%** of total cellular RNA. *Contains 50-60 nucleotides* - Transfer RNA molecules are relatively small, typically containing between **70 to 90 nucleotides**, not 50-60. - This specific length is important for their characteristic **cloverleaf secondary structure** and L-shaped tertiary structure. *Longest RNA* - **Messenger RNA (mRNA)** molecules are generally the longest type of RNA, varying greatly in length depending on the protein they encode. - tRNA molecules are among the **shortest** RNA molecules.

Q: To which site of the ribosome does aminoacyl tRNA attach during protein synthesis?

A site of the ribosome. **A site of the ribosome** - The **A (aminoacyl) site** is where the **incoming aminoacyl-tRNA** first binds to the ribosome during protein synthesis, carrying the next amino acid to be added to the polypeptide chain. - This binding is guided by the **codon-anticodon interaction** between the mRNA at the A site and the anticodon on the tRNA. *P site of the ribosome* - The **P (peptidyl) site** is where the **tRNA carrying the growing polypeptide chain** resides. - After the new amino acid is added from the A site, the tRNA from the A site translocates to the P site. *E site of the ribosome* - The **E (exit) site** is where the **deacylated tRNA** (which has released its amino acid) is released from the ribosome. - It is the final stop for tRNA before it leaves the ribosome to be recharged with another amino acid. *mRNA binding site* - The **mRNA binding site** refers to the general region on the ribosome where the messenger RNA molecule associates. - While the mRNA provides the codons for protein synthesis, it is not the specific site where the **aminoacyl-tRNA** attaches.

Q: Bond formation between ribose sugar and nitrogen is ?

Glycosidic linkage. ***Glycosidic linkage*** - A **glycosidic bond** forms between the **anomeric carbon** of a carbohydrate (like **ribose**) and another functional group (like the **nitrogenous base**). - Specifically, this bond links the **1' carbon atom** of the ribose sugar to a **nitrogen atom** (N-1 in pyrimidines, N-9 in purines) of the nitrogenous base to form a **nucleoside**. *Phosphodiester linkage* - This bond connects the **5' carbon** of one sugar to the **3' carbon** of another sugar via a **phosphate group**, forming the backbone of **DNA and RNA**. - It involves a **phosphate** and **two ester bonds**, not directly linking sugar to a nitrogenous base. *Phosphoester linkage* - A **phosphoester bond** is formed when a **phosphate group** reacts with a **hydroxyl group** of an alcohol, typically found in molecules like **nucleotides** (e.g., 5'-phosphate of a nucleoside). - This type of bond is part of the **phosphodiester linkage** but does not describe the bond between the sugar and the nitrogenous base. *Acidanhydride linkage* - An **acid anhydride linkage** is formed between **two acid groups** with the elimination of water, such as in **ATP** where two phosphate groups are linked by this high-energy bond. - This type of bond is not involved in the connection between a **ribose sugar** and a **nitrogenous base**.

Q: All of the following are true about Nucleic Acid Sequence Based Amplification except which of the following?

Denaturation is carried out at 94°C. ***Denaturation is carried out at 94°C*** - This statement is **incorrect** because **Nucleic Acid Sequence Based Amplification (NASBA)** is an **isothermal** amplification method, meaning it does not require high-temperature denaturation steps. - NASBA operates at a constant temperature (typically around **41°C**), making it distinct from PCR which involves thermal cycling with high-temperature denaturation at 94°C. - This is the **EXCEPT answer** - the only false statement among the options. *It is a specific amplification of RNA* - NASBA is indeed designed for the **specific amplification of RNA targets**, making it particularly useful for detecting RNA viruses or mRNA transcripts. - It utilizes **T7 RNA polymerase** to produce multiple copies of RNA from an RNA template. *It is not a replacement for reverse transcriptase PCR.* - While both detect RNA, NASBA is an **alternative to**, not a replacement for, **reverse transcriptase PCR (RT-PCR)**. - NASBA has advantages in certain settings, such as **isothermal operation** and continuous RNA amplification, but RT-PCR remains the gold standard for many applications. *It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.* - This statement is **correct**. NASBA employs **three key enzymes** in its amplification process: - **Reverse transcriptase (AMV-RT)**: Synthesizes cDNA from RNA template - **RNase H**: Degrades the RNA strand in RNA-DNA hybrids - **T7 RNA polymerase**: Generates multiple RNA copies from the DNA template

Q: What type of linkage connects individual nucleotides in a polynucleotide chain?

3'-5' Phosphodiester linkage. ***3'-5' Phosphodiester linkage*** - This is the correct linkage that connects individual nucleotides in a polynucleotide chain. - The phosphodiester bond forms between the **3'-hydroxyl (OH) group** of the pentose sugar of one nucleotide and the **5'-phosphate group** of the adjacent nucleotide. - This creates the **sugar-phosphate backbone** of DNA and RNA, providing structural integrity and directionality to the polynucleotide chain. - The linkage is named **3'-5'** because it connects the **3' carbon** of one sugar to the **5' carbon** of the next sugar via a phosphate group. *5'-3' Phosphodiester linkage* - This term is **misleading** when describing the linkage itself. - While DNA synthesis and chain reading occur in the **5' to 3' direction**, the actual chemical bond is a **3'-5' phosphodiester linkage**. - The 5'-3' notation refers to the **direction of chain growth**, not the name of the linkage connecting nucleotides. *N-glycosidic linkage* - This bond connects the **nitrogenous base** to the **1' carbon** of the pentose sugar (ribose or deoxyribose). - It is essential for forming a nucleoside and nucleotide, but does **not** link one nucleotide to another in the polynucleotide chain. - This is an intramolecular bond within a single nucleotide, not an internucleotide linkage. *N-glycosidic bond* - This is the same as N-glycosidic linkage—it connects the **base to the sugar** within a single nucleotide. - It does not connect adjacent nucleotides together in a polynucleotide chain. - The bond is between the N9 of purines or N1 of pyrimidines and the C1' of the sugar.

Question 1

What enzyme deficiency is associated with the overproduction of uric acid and the development of gout?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase

Answer

Adenosine deaminase

Answer

Xanthine oxidase

Answer

Uricase

Question 2

Which of the following is NOT a function of DNA polymerase I?

Accepted Answer

Not essential for bacterial survival.

Answer

Involved in the synthesis of Okazaki fragments.

Answer

Repairs damaged DNA.

Answer

Participates in DNA replication.

Question 3

Which of the following statements regarding mitochondrial DNA is FALSE?

Accepted Answer

All respiratory proteins are synthesized within the mitochondria

Answer

Double stranded

Answer

Inherited from mother

Answer

High mutation rate

Question 4

Which of the following statements about tRNA is correct?

Accepted Answer

The CCA sequence is added post-transcriptionally.

Answer

80% of total RNA

Answer

Contains 50-60 nucleotides

Answer

Longest RNA

Question 5

To which site of the ribosome does aminoacyl tRNA attach during protein synthesis?

Accepted Answer

A site of the ribosome

Answer

mRNA binding site

Answer

P site of the ribosome

Answer

E site of the ribosome

Question 6

Bond formation between ribose sugar and nitrogen is ?

Accepted Answer

Glycosidic linkage

Answer

Phosphodiester linkage

Answer

Phosphoester linkage

Answer

Acid anhydride linkage

Question 7

All of the following are true about Nucleic Acid Sequence Based Amplification except which of the following?

Accepted Answer

Denaturation is carried out at 94°C

Answer

It is not a replacement for reverse transcriptase PCR.

Answer

It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.

Answer

It is a specific amplification of RNA

Question 8

Which of the following statements about purine synthesis is true?

Accepted Answer

IMP is the first nucleotide synthesized during purine synthesis.

Answer

Glutamine PRPP amidotransferase is the rate-limiting enzyme in purine synthesis.

Answer

THFA derivatives are required for the formation of C2 and C8 in the purine ring.

Answer

Glutamine donates the amino group for N1 of the purine ring.

Question 9

What type of linkage connects individual nucleotides in a polynucleotide chain?

Accepted Answer

3'-5' Phosphodiester linkage

Answer

5'-3' Phosphodiester linkage

Answer

N-glycosidic linkage

Answer

N-glycosidic bond

Question 10

Which of the following statements about pyrimidine catabolism is correct?

Accepted Answer

β-aminoisobutyrate is produced during the process.

Answer

It directly contributes to carnosine synthesis.

Answer

It generates intermediates of the citric acid cycle.

Answer

It is the primary source of uric acid production.

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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